Gear pump

ABSTRACT

A gear pump having a suction port, first and second discharge ports, an inner rotor having external teeth, an outer rotor having internal teeth such that the number of the internal teeth is larger than that of the external teeth of the inner rotor, and inter-tooth chambers defined by the external teeth and the internal teeth. The first and second discharge ports are in communication with the inter-tooth chamber whose volume is decreased along with rotation of the inner rotor etc. An inter-tooth chamber which has been brought out of communication with the second discharge port is brought into communication with the suction port while the volume is decreasing. The volume of the inter-tooth chamber which has been brought out of communication with the second discharge port is increased after at least a part of the inter-tooth chamber has been brought into communication with the suction port.

TECHNICAL FIELD

The present disclosure relates to a gear pump that includes an innerrotor having a plurality of external teeth and an outer rotor having aplurality of internal teeth and disposed eccentrically with respect tothe inner rotor.

BACKGROUND ART

There has hitherto been known a gear pump that includes an inner rotorhaving n external teeth, an outer rotor having (n+1) internal teethmeshed with the external teeth, and a casing formed with a suction portand a discharge port (see Patent Document 1, for example). In the gearpump, a first angle formed by a first line that connects between thecenter of rotation of the inner rotor and the tooth tip of the externalteeth and a second line that connects between the center of rotation anda meshing portion of the external teeth is 1.4 times or more and 1.8times or less a second angle formed by a third line that connectsbetween the center of rotation and the tooth bottom of the externalteeth and the second line. The width of the external teeth at themeshing portion along the rotational direction is equivalent to thedistance between the rear end, in the rotational direction of therotors, of the suction port and the front end, in the rotationaldirection, of the discharge port, that is, the width of a partitionbetween the ports.

RELATED-ART DOCUMENTS Patent Documents

[Patent Document 1] Japanese Patent No. 4889981

SUMMARY OF THE DISCLOSURE

Patent Document 1 teaches that it is possible to prevent occurrence of aso-called fluid containment, in which a cell with the smallest volume,among a plurality of cells (inter-tooth chambers), positioned at ameshing position, at which the rotors are meshed with each other and arotational drive force is transferred from the external teeth to theinternal teeth, is tightly closed, by determining the first angle as 1.4times or more and 1.8 times or less the second angle. Even if theconfiguration described in Patent Document 1 is adopted, however, it isstill difficult to completely suppress an inflow of a fluid into thecell with the smallest volume, from which the fluid has been dischargedto the discharge port, through a gap between the case and the rotors (agap in the axial direction of the gear pump). Thus, with the gear pumpaccording to Patent Document 1, cavitation may be caused because of arapid inflow of the fluid into the inter-tooth chamber, which is broughtout of communication with the discharge port and which is brought intocommunication with the suction port, through the gap between the caseand the rotors.

Thus, it is an aspect of the present disclosure to provide a gear pumpthat can suppress well occurrence of cavitation in an inter-toothchamber that is brought out of communication with a discharge port andthat is brought into communication with a suction port.

The present disclosure provides a gear pump including a suction port, adischarge port, an inner rotor having a plurality of external teeth, anouter rotor which has a plurality of internal teeth such that the numberof the internal teeth is larger than that of the external teeth of theinner rotor and which is disposed eccentrically with respect to theinner rotor, and a plurality of inter-tooth chambers defined by theplurality of external teeth and the plurality of internal teeth,characterized in that the discharge port is in communication with theinter-tooth chamber whose volume is decreased along with rotation of theinner rotor and the outer rotor, the inter-tooth chamber which has beenbrought out of communication with the discharge port is brought intocommunication with the suction port while the volume of the inter-toothchamber is decreasing, and the volume of the inter-tooth chamber whichhas been brought out of communication with the discharge port isincreased after at least a part of the inter-tooth chamber has beenbrought into communication with the suction port.

In the gear pump, the inter-tooth chamber which has been brought out ofcommunication with the discharge port is brought into communication withthe suction port while the volume of the inter-tooth chamber isdecreased along with rotation of the inner rotor and the outer rotor.Consequently, a fluid is discharged from the inter-tooth chamber whichhas been brought out of communication with the discharge port to thesuction port with the volume of the inter-tooth chamber decreased alongwith rotation of the inner rotor etc. The volume of the inter-toothchamber which has been brought out of communication with the dischargeport is increased after the inter-tooth chamber has been brought intocommunication with the suction port. That is, the volume of theinter-tooth chamber which has been brought out of communication with thedischarge port becomes smallest after the inter-tooth chamber has beenbrought into communication with the suction port. As a result, a rapidinflow of a fluid into the inter-tooth chamber which has been broughtout of communication with the discharge port from a gap between theinner rotor and the outer rotor and a member that houses the inner rotorand the outer rotor (a gap in the axial direction) can be regulated wellby a fluid that flows out of the inter-tooth chamber to the suctionport. Thus, with the gear pump, it is possible to suppress welloccurrence of cavitation in the inter-tooth chamber which is brought outof communication with the discharge port and which is brought intocommunication with the suction port.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a schematic configuration of a gearpump according to the present disclosure.

FIG. 2 is a diagram illustrating a schematic configuration of externalteeth of an inner rotor included in the gear pump according to thepresent disclosure.

FIG. 3 is a diagram illustrating procedures for forming the externalteeth of the inner rotor included in the gear pump according to thepresent disclosure.

FIG. 4 is a diagram illustrating procedures for forming internal teethof an outer rotor included in the gear pump according to the presentdisclosure.

FIG. 5 is an enlarged view illustrating operation of the gear pumpaccording to the present disclosure.

FIG. 6 is an enlarged view illustrating operation of the gear pumpaccording to the present disclosure.

FIG. 7 is an enlarged view illustrating operation of the gear pumpaccording to the present disclosure.

FIG. 8 is a chart illustrating the relationship between the rotationalangle of the inner rotor about the center of rotation and the volume ofan inter-tooth chamber that is brought out of communication with adischarge port.

FIG. 9 is an enlarged view illustrating operation of a gear pumpaccording to another embodiment of the present disclosure.

PREFERRED EMBODIMENTS

Now, an embodiment of the invention according to the present disclosurewill be described with reference to the drawings.

FIG. 1 is a diagram illustrating a schematic configuration of a gearpump 1 according to an embodiment of the present disclosure. The gearpump 1 illustrated in the drawing is constituted as an oil pump to bemounted on a vehicle (not illustrated), for example, and suctionsworking oil (ATF) stored in an oil pan and pumps the working oil to ahydraulic control device (none of such components is illustrated). Thegear pump 1 includes a pump housing constituted from a pump body fixedto a transmission case of an automatic transmission and a pump coverfastened to the pump body (none of such components is illustrated), forexample, and an inner rotor (drive gear) 2 and an outer rotor (drivengear) 3 that are rotatably disposed in a gear housing chamber (notillustrated) defined by the pump housing. The gear pump 1 may beconstituted as an on-vehicle pump (e.g. an engine oil pump) other thanan oil pump that pumps working oil for a transmission, and may beapplied for use other than an on-vehicle pump.

The inner rotor 2 is fixed to a rotary shaft 4 coupled to a crankshaftof an engine mounted on the vehicle (none of such components isillustrated), and rotationally driven by power applied to the rotaryshaft 4. A plurality of (e.g. eleven in the embodiment) external teeth20 are formed on the outer periphery of the inner rotor 2. Meanwhile, anumber (e.g. twelve in the embodiment) of internal teeth 30 are formedon the inner periphery of the outer rotor 3, the number being largerthan the total number of the external teeth 20 of the inner rotor 2 byone. One or a plurality of the internal teeth 30 of the outer rotor 3positioned on the lower side in FIG. 1 are meshed with correspondingexternal teeth 20 of the inner rotor 2. The outer rotor 3 is rotatablydisposed in the gear housing chamber eccentrically with respect to theinner rotor 2. A plurality of inter-tooth chambers (pump chambers) 5 areeach formed between the inner rotor 2 and the outer rotor 3, basicallyby two adjacent external teeth 20 and two adjacent internal teeth 30.

Consequently, when the inner rotor 2 is rotated in the direction of thethick arrow in FIG. 1 by power from the rotary shaft 4, the outer rotor3 is rotated in the same direction as the inner rotor 2 about a centerof rotation 3 c, which is shifted from a center of rotation 2 c of theinner rotor 2 by an amount of eccentricity e, with some of the pluralityof internal teeth 30 meshed with some of the plurality of external teeth20. When the inner rotor 2 and the outer rotor 3 are rotated, the volumeof each of the inter-tooth chambers 5 is increased (the inter-toothchambers 5 are expanded) along with rotation of the inner rotor 2 etc.in a region on the rear side in the rotational direction (see the thickarrow in FIG. 1) of the inner rotor 2 and the outer rotor 3, that is, aregion mainly on the right half in FIG. 1. When the inner rotor 2 andthe outer rotor 3 are rotated, meanwhile, the volume of each of theinter-tooth chambers 5 is decreased (the inter-tooth chambers 5 arecontracted) along with rotation of the inner rotor 2 etc. in a region onthe front side in the rotational direction of the inner rotor 2 etc.,that is, a region mainly on the left half in FIG. 1.

In the pump housing (not illustrated) of the gear pump 1, a suction port6, a first discharge port 7, and a second discharge port 8 that extendgenerally arcuately are formed. The suction port 6 communicates with(faces) such inter-tooth chambers 5, of the plurality of inter-toothchambers 5 defined by the external teeth 20 and the internal teeth 30,that the volume of each of the inter-tooth chambers 5 is increased alongwith rotation of the inner rotor 2 and the outer rotor 3. The first andsecond discharge ports 7 and 8 are separated by a partition wall 9 to beindependent of each other, and communicate with (face) such inter-toothchambers 5, of the plurality of inter-tooth chambers 5, that the volumeof each of the inter-tooth chambers 5 is decreased along with rotationof the inner rotor 2 and the outer rotor 3. In the embodiment, the firstdischarge port 7 which is positioned on the rear side in the rotationaldirection of the inner rotor 2 etc. serves as a low pressure port, andthe second discharge port 8 which is positioned on the front side in therotational direction serves as a high pressure port.

The first and second discharge ports 7 and 8 may be connected todifferent oil passages, or may be connected to a common oil passage. Thesuction port 6 and the first and second discharge ports 7 and 8 may beformed on both sides (in both of the pump body and the pump cover) inthe axial direction of the inner rotor 2 and the outer rotor 3, or maybe formed on one side (in one of the pump body and the pump cover) inthe axial direction of the inner rotor 2 and the outer rotor 3.Furthermore, the suction port 6 may be formed on one side in the axialdirection of the inner rotor 2 etc., and the first and second dischargeports 7 and 8 may be formed on the other side in the axial direction ofthe inner rotor 2 etc., for example. The first discharge port 7 may beformed on one side in the axial direction of the inner rotor 2 etc., andthe second discharge port 8 may be formed on the other side in the axialdirection of the inner rotor 2 etc.

FIG. 2 is a diagram illustrating a schematic configuration of theexternal teeth 20 of the inner rotor 2. FIG. 3 is a diagram illustratingprocedures for forming the external teeth 20. As illustrated in thedrawings, each of the external teeth 20 of the inner rotor 2 includes atooth tip portion 21 in a convex curve shape, a tooth bottom portion 22in a concave curve shape, a first intermediate portion 23 positionedbetween the tooth tip portion 21 and the tooth bottom portion 22 on thefront side, in the rotational direction (see the thick arrow in FIG. 3)of the inner rotor 2, with respect to the tooth tip portion 21, and asecond intermediate portion 24 positioned between the tooth tip portion21 and the tooth bottom portion 22 on the rear side, in the rotationaldirection of the inner rotor 2, with respect to the tooth tip portion21. As illustrated in the drawings, the external teeth 20 are formed tobe horizontally asymmetric with respect to a tooth shape center line Lcthat passes through a top portion 21 t positioned on the radiallyoutermost side of the tooth tip portion 21 and the center of rotation 2c of the inner rotor 2.

As illustrated in FIG. 3, the tooth tip portion 21 is formed in a convexcurve shape by an epitrochoid curve (a portion other than a loopportion). The trochoid coefficient of the curve which is obtained bydividing a radius rde of a first drawn point by a radius re of an outerrolling circle Co is larger than a value of 1 (e.g. a value of about1.2). The epitrochoid curve which forms the tooth tip portion 21 isobtained by keeping the radius rde of the first drawn point at a firstvalue Rde (constant value) and rolling, without slipping, the outerrolling circle Co having the radius re which is smaller than the firstvalue Rde while circumscribing a base circle BCt having a center O thatis common to the center of rotation 2 c of the inner rotor 2.

The tooth bottom portion 22 includes an intermediate portion formed by ahypotrochoid curve (a portion other than a loop portion) and two risingportions formed by a curve such as an arc. The trochoid coefficient ofthe hypotrochoid curve which is obtained by dividing a radius rdh of asecond drawn point by a radius rh of an inner rolling circle Ci islarger than a value of 1. The hypotrochoid curve which forms theintermediate portion of the tooth bottom portion 22 has the base circleBCt in common with the epitrochoid curve which forms the tooth tipportion 21, and is obtained by keeping the radius rdh of the seconddrawn point at a second value Rdh (constant value) and rolling, withoutslipping, the inner rolling circle Ci having the radius rh which issmaller than the second value Rdh while inscribing the base circle BCt.In the embodiment, the radius rde of the first drawn point, that is, thefirst value Rde, for drawing the epitrochoid curve which forms the toothtip portion 21 and the radius rdh of the second drawn point, that is,the second value Rdh, for drawing the hypotrochoid curve which forms thetooth bottom portion 22 are determined as the same value Rd. Similarly,the radius re of the outer rolling circle Co and the radius rh of theinner rolling circle Ci are also determined as the same value R. Thus,the relations Rde=Rdh=Rd, re=rh=R, and tooth height=Rde+re+Rdh+rh=2·eare met for the inner rotor 2.

The two rising portions of the tooth bottom portion 22 extend from theintermediate portion, which is formed by a hypotrochoid curve, towardthe first and second intermediate portions 23 and 24 so as to besmoothly continuous with the intermediate portion. The rising portion onthe rear side in the rotational direction of the inner rotor 2 is formedto be smoothly continuous with the first intermediate portion 23 at anend portion 23 f of the first intermediate portion 23 on the front sidein the rotational direction. The rising portion on the front side in therotational direction of the inner rotor 2 is formed to be smoothlycontinuous with the second intermediate portion 24 at an end portion 24r of the second intermediate portion 24 on the rear side in therotational direction. Consequently, the intermediate portion, which isformed by a hypotrochoid curve, of the tooth bottom portion 22 is offsettoward the center O (the center of rotation 2 c of the inner rotor 2)with respect to the base circle BCt. Furthermore, the tooth bottomportion 22 includes an intersection portion 22 x with a line segment Leobtained by rotating the tooth shape center line Lc forward or rearwardin the rotational direction by half (ϕ/2) an angle ϕ (360°/number of theexternal teeth 20) corresponding to one external tooth 20. In the innerrotor 2, as illustrated in FIGS. 2 and 3, a range between twointersection portions 22 x that interpose the tooth shape center line Lcis determined as a range corresponding to one external tooth 20.

As illustrated in FIGS. 2 and 3, the first intermediate portion 23 isformed between the tooth tip portion 21 and the tooth bottom portion 22on the front side of the tooth tip portion 21 in the rotationaldirection of the inner rotor 2. In the embodiment, the firstintermediate portion 23 is formed by an involute curve determined suchthat a tangent to an end portion 21 f of the tooth tip portion 21 on thefront side in the rotational direction is the same as a tangent to theepitrochoid curve at the end portion 21 f. Consequently, the tooth tipportion 21 and the first intermediate portion 23 can be smoothlycontinuous with each other at the end portion 21 f. In the embodiment,the length of the involute curve which forms the first intermediateportion 23, that is, the distance from the end portion 21 f of the toothtip portion 21 to the end portion 23 f of the first intermediate portion23, is determined to be longer than the length of a curve that forms thesecond intermediate portion 24, that is, the distance from an endportion 21 r of the tooth tip portion 21 to the end portion 24 r of thesecond intermediate portion 24.

As illustrated in FIGS. 2 and 3, the second intermediate portion 24 isformed between the tooth tip portion 21 and the tooth bottom portion 22on the rear side of the tooth tip portion 21 in the rotational directionof the inner rotor 2. The second intermediate portion 24 includes anouter intermediate portion 24 o positioned on the tooth tip portion 21side with respect to an intersection portion 24 x with the base circleBCt, and an inner intermediate portion 24 i positioned on the toothbottom portion 22 side with respect to the intersection portion 24 x. Inthe embodiment, as illustrated in FIG. 3, the outer intermediate portion24 o, that is, a range from the intersection portion 24 x to the endportion (boundary) 21 r of the tooth tip portion 21 on the rear side inthe rotational direction of the inner rotor 2, is formed by a firstcurve obtained by rolling, without sliding, the outer rolling circle Cowhich circumscribes the base circle BCt while varying the radius (seethe dotted line in the drawing) of the first drawn point. Meanwhile, asillustrated in FIG. 3, the inner intermediate portion 24 i, that is, arange from the intersection portion 24 x to the end portion 24 r, isformed by a second curve obtained by rolling, without sliding, the innerrolling circle Ci, which inscribes the base circle BCt, while varyingthe radius (see the dash-double-dot line in the drawing) of the seconddrawn point. See Japanese Patent Application Publication No. 2014-181619(JP 2014-181619 A) for procedures for varying the radius of the first orsecond drawn point of the outer rolling circle Co or the inner rollingcircle Ci.

FIG. 4 is a diagram illustrating procedures for forming the internalteeth 30 of the outer rotor 3 included in the gear pump 1. Asillustrated in the drawing, the tooth shape (contour) of the outer rotor3, which is defined by the plurality of internal teeth 30, is determinedon the basis of an envelope drawn for a plurality of tooth shape lines(the contour of the inner rotor 2; see the dash-double-dot line in FIG.3) obtained by revolving the center of rotation 2 c of an inner rotor2Z, which is based on the inner rotor 2, around by a predetermined angleδ at a time on a circumference having a diameter of 2·e+t and centeredon the center of rotation 3 c of the outer rotor 3, and rotating theinner rotor 2Z by a rotational angle δ/N while the center of rotation 2c is revolved by the predetermined angle δ. It should be noted, however,that “t” is the clearance (tip clearance) between the top portion 21 tof the tooth tip portion 21 of the external teeth 20 and the top portionof the tooth tip portion of the internal teeth 30 at the time when thecenter of rotation 2 c of the inner rotor 2Z, the center of rotation 3 cof the outer rotor 3, the top portion 21 t, and the top portion of theinternal teeth 30 are positioned on one line, and has a value of about0.03 to 0.07 mm, for example.

The inner rotor 2Z for determining the tooth shape of the outer rotor 3corresponds to the inner rotor 2 in which the tooth bottom portion 22has been replaced with a tooth bottom portion 22 z indicated by thedash-double-dot line in FIGS. 2 and 3. As illustrated in FIGS. 2 and 3,the tooth bottom portion 22 z includes a portion that is from the endportion 24 r of the second intermediate portion 24 to a boundary portion22 y indicated in FIGS. 2 and 3 and formed by a hypotrochoid curve (aportion other than a loop portion) that is the same as the hypotrochoidcurve which forms the intermediate portion of the tooth bottom portion22, and a portion that is from the boundary portion 22 y to the endportion 23 f of the first intermediate portion 23 and formed by a smoothcurve (e.g. an arc). Consequently, it is possible to easily obtain theouter rotor 3 which can be adequately meshed with the inner rotor 2. Itshould be noted, however, that the tooth shape (contour) of the outerrotor 3 may be the envelope itself, or may be determined to bepositioned on the outer side with respect to the envelope. The internalteeth of the outer rotor 3 may be formed using a gear cutting having ashape that is generally the same as that of the inner rotor 2Z.

In the gear pump 1, the inner rotor 2 (specifications of the externalteeth 20), the outer rotor 3, the suction port 6, and the first andsecond discharge ports 7 and 8 are configured such that an inter-toothchamber 5 x (see FIG. 1) that has been brought out of communication withthe second discharge port 8 communicates with the suction port 6 whilethe volume of the inter-tooth chamber 5 x is decreasing and the volumeof the inter-tooth chamber 5 x is increased after at least a part of theinter-tooth chamber 5 x and the suction port 6 has been brought intocommunication with each other. In the gear pump 1, additionally, theplurality of external teeth 20 of the inner rotor 2 are formed suchthat, while one of the external teeth 20 that is the closest to the topdead center (a position at which the top portion of the tooth tipportion 21 of the external teeth 20 and the top portion of the tooth tipportion of the internal teeth 30 face (contact) each other on a line)contacts a corresponding one of the internal teeth 30, the externaltooth 20 which is positioned on the rear side, in the rotationaldirection, of the one external tooth 20 by one tooth contacts acorresponding one of the internal teeth 30. By determining thespecifications of the tooth tip portion 21, the tooth bottom portion 22,the first and second intermediate portions 23 and 24, etc. so as to meetsuch conditions, it is possible to suppress well occurrence ofcavitation in the inter-tooth chamber 5 (5 x), and to reduce vibrationor noise by stabilizing the behavior of the inner rotor 2 and the outerrotor 3 during operation of the gear pump 1.

Next, operation of the gear pump 1 will be described with reference toFIGS. 5 to 8. FIGS. 5 to 7 are each an enlarged view illustratingoperation of the gear pump 1. FIG. 8 is a chart illustrating therelationship between a rotational angle θ of the inner rotor 2 about thecenter of rotation 2 c and a volume V of the inter-tooth chamber 5 xwhich is brought out of communication with the second discharge port 8.The rotational angle θ of the inner rotor 2 is a rotational angle, aboutthe center of rotation 2 c, of a line portion that connects between thebottommost portion (deepest portion) of the tooth bottom portion 22 of acertain external tooth 20 and the center of rotation 2 c, and ismeasured counterclockwise in FIG. 1 with the state in which thebottommost portion of the tooth bottom portion 22 of the external tooth20 is positioned directly below the center of rotation 2 c of the innerrotor 2 in the drawing defined as 0°.

In the gear pump 1, the volume V of each of the inter-tooth chambers 5in communication with the second discharge port 8 is decreased alongwith rotation of the inner rotor 2 and the outer rotor 3. When therotational angle θ of the inner rotor 2 reaches a first angle θ1 (seeFIG. 8), as illustrated in FIG. 5, a meshing portion E between theexternal tooth 20 on the rear side in the rotational direction and theinternal tooth 30 which define the inter-tooth chamber 5 x which was incommunication with the second discharge port 8 overlaps a peripheraledge 8 e of the second discharge port 8 as seen in the axial directionof the inner rotor 2, so that communication between the inter-toothchamber 5 x and the second discharge port 8 is blocked. Furthermore,when the inter-tooth chamber 5 x has been brought out of communicationwith the second discharge port 8 with the meshing portion E overlappingthe peripheral edge 8 e of the second discharge port 8, as illustratedin FIG. 5, a tooth surface (the tooth bottom portion 22 or the secondintermediate portion 24) of the external tooth 20 which is on the frontside, in the rotational direction of the inner rotor 2, by one toothwith respect to the external tooth 20 including the meshing portion Eslightly rides on a peripheral edge 6 e of the suction port 6 as seen inthe axial direction of the inner rotor 2. Consequently, the inter-toothchamber 5 x is brought into communication with the suction port 6substantially at the same time as the inter-tooth chamber 5 x is broughtout of communication with the second discharge port 8.

After the inter-tooth chamber 5 x has been brought out of communicationwith the second discharge port 8 and has been brought into communicationwith the suction port 6, as illustrated in FIG. 8, the volume V of theinter-tooth chamber 5 x is further decreased along with rotation of theinner rotor 2 and the outer rotor 3. As illustrated in FIG. 6, the areaof communication between the inter-tooth chamber 5 x and the suctionport 6 as seen in the axial direction of the inner rotor 2 is graduallyincreased along with rotation of the inner rotor 2 and the outer rotor3. In the embodiment, further, when the rotational angle θ of the innerrotor 2 reaches a second angle θ2 (see FIG. 8), as illustrated in FIGS.7 and 8, the entire inter-tooth chamber 5 x is communicated with thesuction port 6 (the entire inter-tooth chamber 5 x overlaps the suctionport 6 as seen in the axial direction), and the volume V of theinter-tooth chamber 5 x reaches a minimum value Vmin.

When the volume of the inter-tooth chamber 5 x has reached the minimumvalue Vmin, as illustrated in FIG. 7, the tooth bottom portion 22between the two external teeth 20 that define the inter-tooth chamber 5x approximates to (substantially contacts) an inner peripheral edge 6 ieof the suction port 6 without projecting toward the center of rotation 2c from the inner peripheral edge 6 ie as seen in the axial direction ofthe inner rotor 2. After the volume V of the inter-tooth chamber 5 x hasreached the minimum value Vmin, as illustrated in FIG. 8, the volume Vof the inter-tooth chamber 5 x is increased along with rotation of theinner rotor 2 and the outer rotor 3, so that working oil is suctionedfrom the suction port 6 into the inter-tooth chamber 5 x.

In the gear pump 1, as discussed above, the inter-tooth chamber 5 xwhich has been brought out of communication with the second dischargeport 8 is brought into communication with the suction port 6 while thevolume V of the inter-tooth chamber 5 x is decreased along with rotationof the inner rotor 2 and the outer rotor 3. Consequently, working oilremaining in the inter-tooth chamber 5 x is discharged to the suctionport 6 as the volume V of the inter-tooth chamber 5 x which has beenbrought out of communication with the second discharge port 8 isdecreased along with rotation of the inner rotor 2 etc. The volume V ofthe inter-tooth chamber 5 x which has been brought out of communicationwith the second discharge port 8 starts increasing after the inter-toothchamber 5 x is fully communicated with the suction port 6. That is, thevolume V of the inter-tooth chamber 5 x which has been brought out ofcommunication with the second discharge port 8 reaches the minimum valueVmin after the inter-tooth chamber 5 x is fully communicated with thesuction port 6.

As a result, a rapid inflow of working oil (leaked oil) into theinter-tooth chamber 5 x which has been brought out of communication withthe second discharge port 8 and which is narrow from a gap between theinner rotor 2 and the outer rotor 3 and at least one of the pump bodyand the pump housing (a gap in the axial direction of the inner rotor 2)can be regulated well by working oil that flows out of the inter-toothchamber 5 x to the suction port 6. Thus, with the gear pump 1, it ispossible to suppress well occurrence of cavitation in the inter-toothchamber 5 x which is brought out of communication with the seconddischarge port 8 and which is brought into communication with thesuction port 6.

In the gear pump 1, the inter-tooth chamber 5 x whose volume V isdecreased along with rotation of the inner rotor 2 and the outer rotor 3is brought out of communication with the second discharge port 8 whenthe meshing portion E between the external tooth 20 and the internaltooth 30 which define the inter-tooth chamber 5 x overlaps theperipheral edge 8 e of the second discharge port 8 as seen in the axialdirection of the inner rotor 2. In the gear pump 1, when the meshingportion E overlaps the peripheral edge 8 e of the second discharge port8 as seen in the axial direction of the inner rotor 2, a tooth surface(the second intermediate portion 24 or the tooth bottom portion 22) ofthe external tooth 20 which is on the front side, in the rotationaldirection of the inner rotor 2, by one tooth with respect to theexternal tooth 20 including the meshing portion E overlaps theperipheral edge 6 e of the suction port 6 as seen in the axialdirection. Consequently, the inter-tooth chamber 5 x whose volume V isdecreased along with rotation of the inner rotor 2 etc., can be broughtinto communication with the suction port 6 immediately after theinter-tooth chamber 5 x has been brought out of communication with thesecond discharge port 8 to allow working oil in the inter-tooth chamber5 x to flow out to the suction port 6. Thus, it is possible to regulatesignificantly well a rapid inflow of working oil (leaked oil) into theinter-tooth chamber 5 x, which has been brought out of communicationwith the second discharge port 8 and which is narrow, from a gap betweenthe inner rotor 2 and the outer rotor 3 and at least one of the pumpbody and the pump housing.

In the gear pump 1, further, the volume V of the inter-tooth chamber 5 xwhich has been brought out of communication with the second dischargeport 8 starts increasing after the entire inter-tooth chamber 5 x iscommunicated with the suction port 6. Consequently, the area ofcommunication between the inter-tooth chamber 5 x which has been broughtout of communication with the second discharge port 8 and the suctionport 6 at the time when working oil starts flowing from the suction port6 into the inter-tooth chamber 5 x in accordance with an increase in thevolume V can be prevented from being narrowed. As a result, it ispossible to suppress an increase in flow rate of working oil that flowsfrom the suction port 6 into the inter-tooth chamber 5 x, and tosuppress well occurrence of cavitation that accompanies suctioning ofworking oil into the inter-tooth chamber 5 x.

In the gear pump 1, as discussed above, the intermediate portion, whichis formed by a hypotrochoid curve, of each of the tooth bottom portions22 of the inner rotor 2 is offset toward the center O (center ofrotation 2 c) with respect to the base circle BCt, and the tooth bottomportion 22 is deeper than a tooth bottom portion corresponding to theinternal teeth 30 of the outer rotor 3. Additionally, as illustrated inFIG. 7, the tooth bottom portion 22 between the two external teeth 20that define the inter-tooth chamber 5 x which has been brought out ofcommunication with the second discharge port 8 approximates to the innerperipheral edge 6 ie of the suction port 6 without projecting toward thecenter of rotation 2 c from the inner peripheral edge 6 ie as seen inthe axial direction of the inner rotor 2 when the volume V of theinter-tooth chamber 5 x has reached the minimum value Vmin. As a result,the area of communication between the inter-tooth chamber 5 x which hasbeen brought out of communication with the second discharge port 8 andthe suction port 6 at the time when working oil starts flowing from thesuction port 6 into the inter-tooth chamber 5 x in accordance with anincrease in the volume V can be increased. Thus, it is possible tosuppress an increase in flow rate of working oil that flows from thesuction port 6 into the inter-tooth chamber 5 x, and to suppresssignificantly well occurrence of cavitation that accompanies suctioningof working oil into the inter-tooth chamber 5 x.

In the gear pump 1, further, the tooth tip portion 21 of each of theexternal teeth 20 of the inner rotor 2 is formed by a portion of anepitrochoid curve whose trochoid coefficient is larger than a value of 1and which is other than a loop portion. Additionally, the tooth bottomportion 22 of the inner rotor 2 is formed by a portion of a hypotrochoidcurve whose the base circle BCt is common to the epitrochoid curve andwhose trochoid coefficient of is larger than a value of 1, and which isother than a loop portion. Consequently, by increasing the radii rde andrdh of the first and second drawn points, that is, the first and secondvalues Rde and Rdh, while keeping the radii re and rh of the outerrolling circle Co and the inner rolling circle Ci (« radius of basecircle BCt/number of teeth) small, it is possible to determine the shapeof the tooth tip portion 21 and the tooth bottom portion 22 using asingle base circle BCt, and to easily increase the tooth height of theexternal teeth 20 while keeping the outside diameter of the base circleBCt, that is, the outside diameter of the inner rotor 2, small.

By increasing the tooth height of the external teeth 20 in this way, themeshing portion E (the locus of the meshing portion E indicated by thedotted line in FIGS. 5 to 7) can be shifted toward the rear side, in therotational direction of the inner rotor 2 etc., with respect to a line(see the dash-and-dot line which extends in the up-down direction inFIG. 1) that passes through the center of rotation 2 c of the innerrotor 2 and the center of rotation 3 c of the outer rotor 3.Consequently, it is possible to easily bring the inter-tooth chamber 5 xwhich has been brought out of communication with the second dischargeport 8 into communication with the suction port 6 while the volume V isdecreasing by approximating an end portion of the suction port 6 on therear side in the rotational direction to an end portion of the seconddischarge port 8 on the front side in the rotational direction.

By making the external teeth 20 asymmetric by making the length of acurve that forms the first intermediate portion 23 of the external teeth20 longer than the length of a curve that forms the second intermediateportion 24, the end portion 21 r of the tooth tip portion 21(epitrochoid curve) on the rear side in the rotational direction can beapproximated to the tooth bottom portion 22, and the end portion 21 f ofthe tooth tip portion 21 on the front side in the rotational directioncan be brought closer to the outer side in the radial direction of theinner rotor 2. By approximating the end portion 21 r of the tooth tipportion 21 on the rear side in the rotational direction to the toothbottom portion 22, it is possible to totally reduce the minimum value ofthe clearance between the external teeth 20 and the internal teeth 30which define the inter-tooth chambers 5 in communication with the firstand second discharge ports 7 and 8. By bringing the end portion 21 f ofthe tooth tip portion 21 on the front side in the rotational directioncloser to the outer side in the radial direction of the inner rotor 2,the minimum value of the clearance between the external teeth 20 and theinternal teeth 30 which define the inter-tooth chambers 5 incommunication with the suction port 6 can be totally increased. As aresult, it is possible to reduce the minimum value (discharge-sideclearance) of the clearance between the external teeth 20 and theinternal teeth 30 which overlap the partition wall 9 separating thefirst and second discharge ports 7 and 8 from each other while improvingthe degree of freedom in determining the position of the partition wall9, that is, the distribution ratio of the discharge flow rates from thefirst and second discharge ports 7 and 8. Additionally, it is possibleto suppress well occurrence of cavitation in the inter-tooth chamber 5whose the amount of variation in volume is the largest, by making theminimum value (suction-side clearance) of the clearance in theinter-tooth chamber 5 sufficiently large.

In the gear pump 1, further, the first intermediate portion 23 which ispositioned on the front side, in the rotational direction of the innerrotor 2, of the tooth tip portion 21 is formed by an involute curve.Consequently, it is possible to smoothly mesh the external teeth 20 ofthe inner rotor 2 and the internal teeth 30 of the outer rotor 3 witheach other, and to make the rotational speed ratio between the innerrotor 2 and the outer rotor 3 constant. It should be understood,however, that the first intermediate portion 23 may be formed by a curveother than an involute curve, such as an n-th order function (n is aninteger having a value of 1 or more), an arc, a desired polynomial, atrigonometric function, an easement curve, or a combination thereof.

It should be understood, however, that the second intermediate portion24 may also be formed by a curve other than an involute curve, such asan n-th order function (n is an integer having a value of 1 or more), anarc, a desired polynomial, a trigonometric function, an easement curve,or a combination thereof. The gear pump 1 may have a single dischargeport. Furthermore, each of the external teeth 20 of the inner rotor 2may be formed symmetrically with respect to the tooth shape center lineLc. The timing when the inter-tooth chamber 5 x is brought intocommunication with the suction port 6 may be slightly later than thetiming when the inter-tooth chamber 5 x is brought out of communicationwith the second discharge port 8 so that the inter-tooth chamber 5 x isnot in communication with the suction port 6 while the inter-toothchamber 5 x is in communication with the second discharge port 8. Thatis, it is not necessary that the timings should perfectly coincide witheach other. Furthermore, as illustrated in FIG. 9, the inner rotor 2,the second discharge port 8, and the input port 6 may be formed suchthat the inter-tooth chamber 5 x is brought into communication with thesuction port 6 before the meshing portion E between the external tooth20 and the internal tooth 30 which define the inter-tooth chamber 5 xoverlaps the peripheral edge 8 e of the second discharge port 8 as seenin the axial direction of the inner rotor 2. That is, the timing whenthe inter-tooth chamber 5 x is brought into communication with thesuction port 6 may be slightly earlier than the timing when theinter-tooth chamber 5 x is brought out of communication with the seconddischarge port 8 to the extent that the discharge pressure from thesecond discharge port 8 is not significantly affected. Consequently, theinter-tooth chamber 5 x whose the volume is decreased along withrotation of the inner rotor 2 etc., can be brought into communicationwith the suction port 6 before the inter-tooth chamber 5 x is broughtout of communication with the second discharge port 8 to allow anappropriate amount of working oil in the inter-tooth chamber 5 x to flowout to the second discharge port 8 and the suction port 6. As a result,it is possible to hold the pressure of working oil in the inter-toothchamber 5 x so as not to be raised more than necessary, and to suppressoccurrence of vibration due to a rise in pressure of working oil in theinter-tooth chamber 5 x.

As has been described above, the present disclosure provides a gear pump(1) including a suction port (6), a discharge port (7, 8), an innerrotor (2) having a plurality of external teeth (20), an outer rotor (3)which has a plurality of internal teeth (30) such that the number of theinternal teeth (30) is larger than that of the external teeth (20) ofthe inner rotor (2), and which is disposed eccentrically with respect tothe inner rotor (2), and a plurality of inter-tooth chambers (5) definedby the plurality of external teeth (20) and the plurality of internalteeth (30), characterized in that the discharge port (7, 8) is incommunication with the inter-tooth chamber (5) whose volume (V) isdecreased along with rotation of the inner rotor (2) and the outer rotor(3), the inter-tooth chamber (5 x) which has been brought out ofcommunication with the discharge port (8) is brought into communicationwith the suction port (6) while the volume (V) of the inter-toothchamber (5 x) is decreasing, and the volume (V) of the inter-toothchamber (5 x) which has been brought out of communication with thedischarge port (8) is increased after at least a part of the inter-toothchamber (5 x) has been brought into communication with the suction port(6).

In the gear pump, the inter-tooth chamber which has been brought out ofcommunication with the discharge port is brought into communication withthe suction port while the volume of the inter-tooth chamber isdecreased along with rotation of the inner rotor and the outer rotor.Consequently, a fluid is discharged from the inter-tooth chamber whichhas been brought out of communication with the discharge port to thesuction port as the volume of the inter-tooth chamber is decreased alongwith rotation of the inner rotor etc. The volume of the inter-toothchamber which has been brought out of communication with the dischargeport is increased after the inter-tooth chamber has been brought intocommunication with the suction port. That is, the volume of theinter-tooth chamber which has been brought out of communication with thedischarge port becomes smallest after the inter-tooth chamber has beenbrought into communication with the suction port. As a result, a rapidinflow of a fluid into the inter-tooth chamber which has been broughtout of communication with the discharge port from a gap between theinner rotor and the outer rotor and a member that houses the inner rotorand the outer rotor (a gap in the axial direction) can be regulated wellby a fluid that flows out of the inter-tooth chamber to the suctionport. Thus, with the gear pump, it is possible to suppress welloccurrence of cavitation in the inter-tooth chamber which is brought outof communication with the discharge port and which is brought intocommunication with the suction port.

The volume (V) of the inter-tooth chamber (5 x) which has been broughtout of communication with the discharge port (6) may start increasingafter the entire inter-tooth chamber (5 x) is communicated with thesuction port (6). Consequently, the area of communication between theinter-tooth chamber which has been brought out of communication with thedischarge port and the suction port at the time when a fluid startsflowing from the suction port into the inter-tooth chamber in accordancewith an increase in the volume can be prevented from being narrowed. Asa result, it is possible to suppress an increase in flow rate of a fluidthat flows from the suction port into the inter-tooth chamber, and tosuppress well occurrence of cavitation that accompanies suctioning ofthe fluid into the inter-tooth chamber.

The inner rotor (2) may be formed such that a tooth bottom portion (22)that defines the inter-tooth chamber (5 x) which has been brought out ofcommunication with the discharge port (8) approximates to an innerperipheral edge (6 ie) of the suction port (6) without projecting fromthe inner peripheral edge (6 ie) toward a center of rotation (2 c) ofthe inner rotor (2) as seen in an axial direction of the inner rotor (2)when the volume (V) of the inter-tooth chamber (5 x) has becomesmallest. Consequently, the minimum volume of the inter-tooth chamber,that is, the area of communication between the inter-tooth chamber whichhas been brought out of communication with the discharge port and thesuction port at the time when a fluid starts flowing from the suctionport into the inter-tooth chamber in accordance with an increase in thevolume, can be increased. As a result, it is possible to suppress anincrease in flow rate of a fluid that flows from the suction port intothe inter-tooth chamber, and to suppress significantly well occurrenceof cavitation that accompanies suctioning of the fluid into theinter-tooth chamber. In this case, by deepening the tooth bottom portionof the external teeth of the inner rotor (offsetting the tooth bottomportion toward the center of rotation of the inner rotor), for example,the tooth bottom portion can be approximated to the inner peripheraledge of the suction port when the volume of the inter-tooth chamberreaches the minimum value, and the minimum volume (area ofcommunication) of the inter-tooth chamber which has been brought out ofcommunication with the discharge port can be increased adequately.

The inner rotor (2) may be formed such that the inter-tooth chamber (5x) whose volume (V) is decreased is brought into communication with thesuction port (6) after a meshing portion (E) between the external tooth(20) and the internal tooth (30) defining the inter-tooth chamber (5 x)overlaps a peripheral edge (8 e) of the discharge port (8) as seen in anaxial direction of the inner rotor (2). Consequently, the inter-toothchamber whose volume is decreased along with rotation of the inner rotoretc. can be brought into communication with the suction portsubstantially at the same time as the inter-tooth chamber has beenbrought out of communication with the discharge port to allow a fluid inthe inter-tooth chamber to flow out to the suction port. Thus, it ispossible to regulate significantly well an inflow of the fluid into theinter-tooth chamber which has been brought out of communication with thedischarge port from a gap between the inner rotor and the outer rotorand a member that houses the inner rotor and the outer rotor.

The inner rotor (2) may be formed such that the inter-tooth chamber (5x) whose volume (V) is decreased is brought into communication with thesuction port (6) before a meshing portion (E) between the external tooth(20) and the internal tooth (30) defining the inter-tooth chamber (5 x)overlaps a peripheral edge (8 e) of the discharge port (8) as seen in anaxial direction of the inner rotor (2). Consequently, the inter-toothchamber whose volume is decreased along with rotation of the inner rotoretc. can be brought into communication with the suction port before theinter-tooth chamber is brought out of communication with the dischargeport to allow an appropriate amount of fluid in the inter-tooth chamberto flow out to the discharge port and the suction port. As a result, itis possible to hold the pressure of a fluid in the inter-tooth chamberso as not to be raised more than necessary, and to suppress occurrenceof vibration due to a rise in pressure of the fluid in the inter-toothchamber.

Each of the external teeth (20) of the inner rotor (2) may include atooth tip portion (21) formed by an epitrochoid curve obtained byrolling, without sliding, an outer rolling circle (Co) having a radius(re) that is smaller than a radius (rde) of a drawn point whilecircumscribing a base circle (BCt). That is, by increasing the radius ofa drawn point of the epitrochoid curve while keeping the radius of theouter rotating circle (∝ radius of base circle/number of teeth) small,it is possible to easily increase the tooth height of the external teethwhile keeping the outside diameter of the base circle, that is, theoutside diameter of the inner rotor, small. By increasing the toothheight of the external teeth, the meshing portion (the locus of themeshing portion) between the external teeth and the internal teeth canbe shifted rearward in the rotational direction of the inner rotor etc.with respect to a line that passes through the center of rotation of theinner rotor and the center of rotation of the outer rotor. Consequently,it is possible to easily bring the inter-tooth chamber which has beenbrought out of communication with the discharge port into communicationwith the suction port while the volume is decreasing by approximating anend portion of the suction port on the rear side in the rotationaldirection to an end portion of the discharge port on the front side inthe rotational direction of the inner rotor etc.

Each of the external teeth (20) of the inner rotor (2) may include afirst intermediate portion (23) formed by a desired curve and positionedbetween the tooth tip portion (21) and a tooth bottom portion (22)positioned on a front side, in a rotational direction of the inner rotor(2), with respect to the tooth tip portion (21), and a secondintermediate portion (24) formed by a desired curve and positionedbetween the tooth tip portion (21) and a tooth bottom portion (22)positioned on a rear side, in the rotational direction of the innerrotor (2), with respect to the tooth tip portion (21); and a length ofthe curve which forms the first intermediate portion (23) may be longerthan a length of the curve which forms the second intermediate portion(24).

By making the external teeth asymmetric by making the length of a curvethat forms the first intermediate portion longer than the length of acurve that forms the second intermediate portion in this way, the endportion of the epitrochoid curve which forms the tooth tip portion onthe rear side in the rotational direction can be approximated to thetooth bottom portion, and the end portion of the epitrochoid curve onthe front side in the rotational direction can be brought closer to theouter side in the radial direction of the inner rotor. By approximatingthe end portion of the epitrochoid curve which forms the tooth tipportion on the rear side in the rotational direction to the tooth bottomportion, it is possible to totally reduce the minimum value of theclearance between the external teeth and the internal teeth which definethe inter-tooth chambers in communication with the discharge port. Bybringing the end portion of the epitrochoid curve which forms the toothtip portion on the front side in the rotational direction closer to theouter side in the radial direction of the inner rotor, the minimum valueof the clearance between the external teeth and the internal teeth whichdefine the inter-tooth chambers in communication with the suction portcan be totally increased.

The first intermediate portion (23) may be formed by at least aninvolute curve. Consequently, it is possible to smoothly mesh theexternal teeth and the internal teeth with each other, and to make therotational speed ratio between the inner rotor and the outer rotorconstant.

The discharge port may include a first discharge port (7) and a seconddischarge port (8) separated from the first discharge port (7) by apartition wall (9) and disposed on a front side, in a rotationaldirection of the inner rotor (2), with respect to the first dischargeport (7).

The present disclosure is not limited to the embodiment described abovein any way, and it is a matter of course that the embodiments may bemodified in various ways without departing from the range of theextension of the present disclosure. Furthermore, the mode for carryingout the invention described above is merely a specific of an embodimentdescribed in the “SUMMARY OF THE DISCLOSURE” section, and does not limitthe elements of the embodiments described in the “SUMMARY OF THEDISCLOSURE” section.

INDUSTRIAL APPLICABILITY

The disclosure according to the present disclosure is applicable to thegear pump manufacturing industry.

1. A gear pump including a suction port, a discharge port, an innerrotor having a plurality of external teeth, an outer rotor which has aplurality of internal teeth such that the number of the internal teethis larger than that of the external teeth of the inner rotor and whichis disposed eccentrically with respect to the inner rotor, and aplurality of inter-tooth chambers defined by the plurality of externalteeth and the plurality of internal teeth, wherein the discharge port isin communication with the inter-tooth chamber, whose volume is decreasedalong with rotation of the inner rotor and the outer rotor, theinter-tooth chamber which has been brought out of communication with thedischarge port is brought into communication with the suction port whilethe volume of the inter-tooth chamber is decreasing, and the volume ofthe inter-tooth chamber which has been brought out of communication withthe discharge port is increased after at least a part of the inter-toothchamber has been brought into communication with the suction port. 2.The gear pump according to claim 1, wherein the volume of theinter-tooth chamber which has been brought out of communication with thedischarge port starts increasing after the entire inter-tooth chamber iscommunicated with the suction port.
 3. The gear pump according to claim1, wherein the inner rotor is formed such that a tooth bottom portionthat defines the inter-tooth chamber which has been brought out ofcommunication with the discharge port approximates to an innerperipheral edge of the suction port without projecting from the innerperipheral edge toward a center of rotation of the inner rotor as seenin an axial direction of the inner rotor when the volume of theinter-tooth chamber has become smallest.
 4. The gear pump according toclaim 1, wherein the inner rotor is formed such that the inter-toothchamber whose volume is decreased is brought into communication with thesuction port after a meshing portion between the external tooth and theinternal tooth defining the inter-tooth chamber overlaps a peripheraledge of the discharge port as seen in an axial direction of the innerrotor.
 5. The gear pump according to claim 1, wherein the inner rotor isformed such that the inter-tooth chamber whose volume is decreased isbrought into communication with the suction port before a meshingportion between the external tooth and the internal tooth defining theinter-tooth chamber overlaps a peripheral edge of the discharge port asseen in an axial direction of the inner rotor.
 6. The gear pumpaccording to claim 1, wherein each of the external teeth of the innerrotor includes a tooth tip portion formed by an epitrochoid curveobtained by rolling, without sliding, an outer rolling circle having aradius that is smaller than a radius of a drawn point whilecircumscribing a base circle.
 7. The gear pump according to claim 6,wherein: each of the external teeth of the inner rotor includes a firstintermediate portion formed by a desired curve and positioned betweenthe tooth tip portion and a tooth bottom portion positioned on a frontside, in a rotational direction of the inner rotor, with respect to thetooth tip portion, and a second intermediate portion formed by a desiredcurve and positioned between the tooth tip portion and a tooth bottomportion positioned on a rear side, in the rotational direction of theinner rotor, with respect to the tooth tip portion; and a length of thecurve which forms the first intermediate portion is longer than a lengthof the curve which forms the second intermediate portion.
 8. The gearpump according to claim 7, wherein the first intermediate portion isformed by at least an involute curve.
 9. The gear pump according toclaim 1, wherein the discharge port includes a first discharge port anda second discharge port separated from the first discharge port by apartition wall and disposed on a front side, in a rotational directionof the inner rotor, with respect to the first discharge port.
 10. Thegear pump according to claim 2, wherein the inner rotor is formed suchthat a tooth bottom portion that defines the inter-tooth chamber whichhas been brought out of communication with the discharge portapproximates to an inner peripheral edge of the suction port withoutprojecting from the inner peripheral edge toward a center of rotation ofthe inner rotor as seen in an axial direction of the inner rotor whenthe volume of the inter-tooth chamber has become smallest.
 11. The gearpump according to claim 2, wherein the inner rotor is formed such thatthe inter-tooth chamber whose volume is decreased is brought intocommunication with the suction port after a meshing portion between theexternal tooth and the internal tooth defining the inter-tooth chamberoverlaps a peripheral edge of the discharge port as seen in an axialdirection of the inner rotor.
 12. The gear pump according to claim 2,wherein the inner rotor is formed such that the inter-tooth chamberwhose volume is decreased is brought into communication with the suctionport before a meshing portion between the external tooth and theinternal tooth defining the inter-tooth chamber overlaps a peripheraledge of the discharge port as seen in an axial direction of the innerrotor.
 13. The gear pump according to claim 2, wherein each of theexternal teeth of the inner rotor includes a tooth tip portion formed byan epitrochoid curve obtained by rolling, without sliding, an outerrolling circle having a radius that is smaller than a radius of a drawnpoint while circumscribing a base circle.
 14. The gear pump according toclaim 13, wherein: each of the external teeth of the inner rotorincludes a first intermediate portion formed by a desired curve andpositioned between the tooth tip portion and a tooth bottom portionpositioned on a front side, in a rotational direction of the innerrotor, with respect to the tooth tip portion, and a second intermediateportion formed by a desired curve and positioned between the tooth tipportion and a tooth bottom portion positioned on a rear side, in therotational direction of the inner rotor, with respect to the tooth tipportion; and a length of the curve which forms the first intermediateportion is longer than a length of the curve which forms the secondintermediate portion.
 15. The gear pump according to claim 14, whereinthe first intermediate portion is formed by at least an involute curve.16. The gear pump according to claim 2, wherein the discharge portincludes a first discharge port and a second discharge port separatedfrom the first discharge port by a partition wall and disposed on afront side, in a rotational direction of the inner rotor, with respectto the first discharge port.
 17. The gear pump according to claim 10,wherein the inner rotor is formed such that the inter-tooth chamberwhose volume is decreased is brought into communication with the suctionport after a meshing portion between the external tooth and the internaltooth defining the inter-tooth chamber overlaps a peripheral edge of thedischarge port as seen in an axial direction of the inner rotor.
 18. Thegear pump according to claim 17, wherein each of the external teeth ofthe inner rotor includes a tooth tip portion formed by an epitrochoidcurve obtained by rolling, without sliding, an outer rolling circlehaving a radius that is smaller than a radius of a drawn point whilecircumscribing a base circle.
 19. The gear pump according to claim 18,wherein: each of the external teeth of the inner rotor includes a firstintermediate portion formed by a desired curve and positioned betweenthe tooth tip portion and a tooth bottom portion positioned on a frontside, in a rotational direction of the inner rotor, with respect to thetooth tip portion, and a second intermediate portion formed by a desiredcurve and positioned between the tooth tip portion and a tooth bottomportion positioned on a rear side, in the rotational direction of theinner rotor, with respect to the tooth tip portion; and a length of thecurve which forms the first intermediate portion is longer than a lengthof the curve which forms the second intermediate portion.
 20. The gearpump according to claim 19, wherein the first intermediate portion isformed by at least an involute curve.